1. Field of the Invention
The present invention relates to a Radio Frequency (RF) filter.
2. Description of the Related Art
A filter is an essential part of an RF system, for passing or rejecting a predetermined frequency band. Filters are classified into Band Pass Filters (BPFs), Band Rejection Filters (BRFs), High Pass Filters (HPFs), Low Pass Filters (LPFs), etc. Such filters are designed so as to satisfy frequency pass or rejection characteristics required for the RF system. Once the filters are installed, their characteristics are difficult to change unless they are replaced. For example, when a BRF is to be reinstalled in an RF system equipped with a BPF, the BPF should be removed and then replaced with the BRE. The filter replacement may lead to communication disconnection and increase cost.
Meanwhile, the BRF is designed by connecting resonators using a 50-ohm (Ω) line. In this case, since no coupling occurs between resonators, a frequency band equal to or wider than a predetermined bandwidth cannot be rejected. In addition, as the number of resonators increases, the total length of the 50-ohm line also increases, thereby increasing path loss. Another shortcoming of the BRF is that a low or high frequency band is selectively rejected with respect to a specific frequency and thus it is difficult to improve band edge characteristics.